Electron beam traverse of narrow aperture in barrier separating regions of differentpressure



c. w. HANKS' 3,299,308

ERTURE IN BARRIER SEPARATING REGIONS OF DIFFERENT PRESSURE Jan; 17, 1967ELECTRON BEAM TRAVERSE OF NARROW AP Filed July 19, 1963 fizz; 1

Char/es W Hanks United States Patent Office 3,299,308 ELECTRON BEAMTRAVERSE F NARROW APER- TURE IN BARRIER SEPATING REGIONS OF DIFFERENTPRESSURE Charles W. Hanks, Orinda, Calif., assignor to TemescalMetallurgical Corporation, Berkeley, Calif, 21 corporation of CaliforniaFiled July 19, 1963, Ser. No. 296,175 Claims. (Cl. 313-160) The presentinvention relates generally to an electron beam apparatus and, moreparticularly, to an improved electron beam apparatus for treatingmaterials.

Electron beam apparatus has been employed for various material treatingprocesses, such as melting, annealing, cleaning, vapor-plating, etc.Normally, an electron beam apparatus includes a source of electrons andfocusing means for forming the electrons into a beam and directing thesame at the material to be treated, the material to be treated and theelectron source being enclosed in an evacuated chamber. Eitherelectrostatic fields or magnetic fields may be employed for focusingelectrons into a beam. The beam electrons bombard the work piece andheat the same. Gases and vapors of various kinds are emitted by theheated material. These gases may cause arc discharges in theelectrostatic fields if they are not immediately removed from thechamber.

In previously available electron beam apparatus, the gases have beenremoved by providing a vacuum pump system which had a sufficientcapacity to handle the maximum gas emitted by the heated metal. Suchpump systems are expensive and bulky.

Various sources of electrons have been provided in the electron beamapparatus. For high power applications, the so-called Pierce guns havebeen employed. A Pierce gun generally includes an emitting surface whichmay be an indirectly heated cathode or a directly heated cathode. Thebeam is produced by the cathode and is formed by a focus electrode andan anode into a desired shape, for example, round, rectangular etc. Theshape of the beam is determined by the shape of apertures in the focuselectrode and the anode. The focus electrode which is at the samepotential as the cathode extends outwardly from the cathode at an angleof about 67 /2 degrees with respect to the axis of the desired beam. Theanode of the gun is spaced from the cathode and extends outwardly fromthe edges of the desired beam at a predetermined angle. The placement ofthe focus electrode and the anode is normally critical.

If the anode is placed too close to the cathode, a divergent beam willbe formed which beam is normally unusable. For more beam current, it isdesirable to place the anode close to the cathode. It has been difficultto provide a gun having a higher perveance, which is equal to the DC.beam current divided by the three half power of the DC. beam voltage,than 0.5 1() and still produce a convergent or parallel beam.

An object of the present invention is the provision of an improvedelectron beam apparatus for treating metal. Another object of theinvention is the provision of an electron beam apparatus which permitsefficient use of high output electron guns. A further object is theprovision of an electron beam apparatus in which relatively small vacuumpumps are employed. A still further object is the provision of anelectron beam apparatus which is economical to manufacture and use andwhich is durable in operation.

Other objects and advantages of the present invention will becomeapparent with reference to the following description and accompanyingdrawings.

In the drawings:

FIGURE 1 is a schematic cross sectional view of one 3,299,308 PatentedJan. 17, 1967 embodiment of the electron beam apparatus showing variousfeatures of the present invention;

FIGURE 2 is an enlarged schematic cross section of an electron gun whichmay be employed in the electron beam apparatus shown in FIGURE 1; and

FIGURE 3 is a schematic cross sectional view of another embodiment of anelectron beam apparatus showing various features of the presentinvention.

The electron beam apparatus shown in FIGURE 1 includes an air-tightenclosure 10 which is divided into three chambers 12, 14 and 16 by apair of spaced barriers 18 and 20. An electron source or gun 22 isdisposed in one end chamber (gun chamber 12) and a target 24 (thematerial to be treated) is disposed in the other end chamber (treatingchamber 16). Each of the barriers 18 and 20 has an aperture 26 and 28,respectively, therein which is in alignment with the axis of a beamproduced by the electron gun 22. Means 30 is provided for establishing auniform magnetic field extending parallel to the axis of the beam ofelectrons whereby electrons from the gun 22 follow helical paths to theapertures 26 and 28. The distance between the barriers 18 and 20 andbetween the barrier 18 and gun 22 is made substantially equal to anintegral number of revolutions of the helical path of the electrons.Pump means 32, 34 and 36 are connected to the chambers 12, 14 and 16,respectively, for evacuating the same.

If an electron moves parallel to a uniform magnetic field, it is notinfluenced and travels along a straight line. If the electron moves in adirection perpendicular to the uniform field, its path is a circle. Ifan electron moves in a direction between that of parallel andperpendicular, its path is helical, the axis of the helix being parallelto the magnetic field. Electrons leaving a point on a magnetic fieldline at the same angle, although in different directions, will arriveback at the field line at the same time and at the same point which isfarther along the field line by a distance equal to a revolution of thehelix (the pitch). The pitch of the helix depends on the cosine of theangle of departure of electrons from the field line. For small anglesthe pitch does not change much with the change of angle. Hence, if amagnetic field is placed parallel to a beam of electrons all movinggenerally in the same direction, the electrons will all return topositions corresponding to their original position in a distance alongthe axis of the beam equal to the pitch of the helix. Thus, an image ofan emitting surface is produced one or more revolutions or pitches ofthe helical path of the electrons along the field line. The length of arevolution or the pitch of the helical path is equal to Where V is theelectron velocity in equivalent volts; B is the magnetic flux density inwebers per square meter, and 0 is the angle of depature from the fieldline.

In the illustrated embodiment, this production of images of the emittingsurface is utilized to reduce the amount of gas or vapors which reachthe electron gun. More specifically, as shown in FIGURE 1, the electronbeam apparatus includes the air-tight enclosure 10 which may be of anysuitable shape such as rectangular, cylindrical, etc. The enclosure 10is divided into three chambers 12, 14 and 16 by the pair of spacedparallel barriers 18 and 20. The barriers may be of any suitablematerial which will withstand the temperature and environmentalconditions of the apparatus.

In FIGURE 1, the central part of each barrier 18 and 20 is formed of acircular plate 38 and 40 of ferromagnetic material, such as steel. Theremainder of the barriers are made of a material, such as copper. Theplates 38 and 40 are provided with central and aligned apertures 26 and28, described more fully hereinafter. Besides serving as part of thebarriers, the plates 38 and 40 also serve as pole pieces for providing auniform magnetic field in the intermediate or transition chamber 14parallel to an axis extending between the apertures 26 and 28. Themagnetic field is generated by a plurality of iron core coils 42extending between the plates 38 and 40 and disposed adjacent theperiphery thereof.

As shown in FIGURE 1, the electron source or gun 22 is disposed in theend or gun chamber 12 in a position such as to direct a beam ofelectrons along the axis of the apertures 26 and 28. The electron gun 22may be any suitable linear type gun but preferably, for high currentoutput, is a Pierce type gun. The electron gun 22 is suitably supportedin spaced relation to the barrier 18 and is disposed in a centralaperture of a pole piece 44 which may be a plate of ferromagneticmaterial similar to the plates 38 and 40.

A uniform field is generated parallel to an axis extending between theelectron gun 22 and the aperture 26 by iron core coils 46 extendingbetween the plates 38 and 44 adjacent the periphery thereof. Thus theelectrons, upon being emitted by the emitting surface of the electrongun 22, are immediately influenced by the uniform field. Consequently,the electrons follow a helical path upon their emission from theemitting surface and form images of the emitting surface at an integralnumber of revolutions or pitches of the helical path.

The spacing between the pole pieces 38 and 40 and the spacing betweenthe emitting surface of the electron gun 22 and the pole piece 38 aremade such that an image of the emitting surface is produced at each ofthe apertures 26 and 28. In this connection, the distance between theapertures 26 and 28 and between the emitting surface and the aperture 26is selected so as to be equal to an integral number of the revolutionsor pitches of the helical path.

As previously indicated, the electron gun 22 is pre- \ferably a Piercetype gun. Such a gun provides a high current beam and thus provides ahigher heating effect on the material 24 to be treated. As shown inFIGURE 2, the Pierce type gun of the illustrated embodiment includes anemitting surface or cathode 48 which is in the form of an elongatedcylinder or filament of a material such as tungsten. The filament isheated by passing current through the same. An indirectly heated cathodemay be employed in certain applications. A pair of plates 50 and 52which serve as the focus electrode extend from the cathode 48 at anangle of 67 /2 relative to the axis of the beam of electrons. The plates50 and 52 are made of a material which will withstand the temperature towhich they are exposed. A suitable material is tungsten or tantalum. Thefocus electrodes may be formed of a single plate with a slot to receivethe cathode. The cathode 48 and the pair of focusing plates 50 and 52are connected to a source of high negative potential 54.

In the illustrated embodiment, the anode of the electron gun is formedbya pair of metal rods 56 and 58 which extend parallel to the cathodeand on either side of the beam of electrons. The rods, which may beloosely supported, are made of a refractory metal, such as tungsten,tantalum, etc. The rods 56 and 58, as well as the pole pieces 44, 38,and 40 and the target 24 are grounded. As previously indicated, theelectrons are constrained and directed by the magnetic field as soon asthey are emitted from the cathode and, hence, the electrons areprevented from diverging. This permits the use of loosely held rods asanodes and the use of a relatively small distance between the anode andcathode and hence a higher beam current. Without the magnetic field,such a construction would produce a divergent beam which would normallybe unusable. Also, the magnetic focusing reduces the amount of anodebombardment by the electron beam and hence, even at high outputs, anodecooling, other than radiation, is not required.

Since an image of the emitting surface is produced at the apertures 26and 28, the apertures 26 and 28 are made of the same shape as theelectron emitting surface and are aligned with the emitting surface.Thus, in the illustrated embodiment, the apertures 26 and 28 are maderectangular since the cathode is rectangular. The edges of the apertures26 and 28 are beveled to prevent the electrons from striking the polepieces 38 and 48. However, the bevel is not critical.

Gas and vapor is evolved by the electron beam impinging on and heatingthe target 24. The barriers 40 and 38 greatly reduce the amount of gasand vapor which enters the gun chamber 12. Gas and vapor can only enterthe gun chamber 12 by passing through the apertures 26 and 28. Theapertures 26 and 28 are made about the size and shape of the emitter andhence a relatively minor amount of leakage occurs between chambers. Theleakage through the aperture 26 is especially minor since at the normaloperating pressures of chambers 12 and 14, so-called molecular flowoccurs through the aperture 26 instead of the normal viscous flow. Inthis connection, the mass flow of gases and vapors through orificesdrops significantly as soon as a pressure is reached where molecularfiowoccurs instead of the viscous flow. This is defined as a pressure suchthat the distance between the sides of the orifice is equal to or lessthan the mean free path of the vapor or gas molecules. The molecularflow of gases through an orifice is about 43% less than that of viscousflow through the orifice. By making the aperture 26 a narrow rectangle,molecular flow occurs at a higher pressure than for a cylindricalaperture of the same area. This results in a considerable saving in thesize of pumps required to maintain a given pressure drop across theaperture, or conversely, it permits an aperture with a larger crosssectional area for a given size pump. 1

The target 24 to be treated, which may be of metal or non-metal, issuitably mounted in the treating chamber 16. The beam emerging from thelast aperture 28 is no longer under the influence of the magnetic fieldand hence, the beam tends to spread, due to space charge repulsion. Theposition of the target relative to the last aperture 28 depends on theoperations to be performed on the target and the amount of powerrequired. The closer the target is to an image point of the beam themore intense the beam will be. For example, to melt the metal, the beamis normally spread over its entire surface to reduce hot spots.

To minimize the amount of ions and metal vapor which enter thetransition chamber and the gun chamber, the target may be positioned outof alignment with the apertures 26 and 28. The electron beam is focusedon the target by a traverse magnetic field provided by a suitable means(not shown) after the last aperture 28, the field bending the electronbeam as it emerges from the last aperture 28. Thus, vapor and ionsproduced by electron bombardment of the target tend to strike thebarrier 40 rather than passing through the aperture 28.

The chambers 12, 14 and 16 are evacuated by suitable pumps 32, 34 and36, respectively. Preferably, for the most stability, the electron gunchamber 12 is evacuated to as high a vacuum as possible, i.e., higherthan about 0.1 micron of Hg. Because of the pressure barrier system ofthe illustrated apparatus, very little vapor or gas from the treatingchamber 16 enters the electron gun chamber 12. Thus a high speeddiffusion pump can handle the low mass vapor flow involved and maintainthe electron gun chamber 12 at a high vacuum.

The pressure at which the treating chamber 16 is operated depends uponthe operation to be performed on the target 24. For normal operations,the treating chamher is preferably evacuated at a speed sufficient toquickly remove the vapor and gases evolved during the operation.

A relatively low vacuum may be tolerated in the treating chamber 16,since high voltage electrostatic fields are not present in the treatingchamber. Thus a higher pressure pump, such as a mechanical vacuum pump,can be employed to evacuate the treating chamber 16.

The transition chamber 14 is evacuated to a vacuum intermediate that ofthe treating chamber :16 and the gun chamber 12. The allowable pressuredifferential between the chambers depends upon the amount of gasesevolved. For lower amounts of gases, a pressure differential of up toabout 100 to 1 can be tolerated. In certain applications, the transitionchamber may be eliminated. For large amounts of evolved gases thepressure differential, preferably is not .greater than to 1. Additionaltransition chambers can be provided to provide the lower differentialpressure across the barrier.

In one illustrated embodiment of the electron beam apparatus the emittersize is A: inch wide by A2 inch long. The apertures in the barriers areA; inch wide by 1 inch 'long. The pole pieces are made of steel and arespaced approximately 8 inches apart. The magnetic field intensity is 108gausses for 10 kv. electrons and 130 gausses for kv. electrons. The gunchamber is evacuated to 0.6 micron of Hg, the transition chamber to 17microns of Hg, and the treating chamber to 100 microns of Hg.

In the embodiment shown in FIGURE 3, wherein parts similar to thoseshown in FIGURE 1 are indicated with the same reference numeral and withthe suffix a, the uniform magnetic field is generated by Helmholtz coils60, 62 and 64. Helmholtz coils are coaxial air core coils which arespaced so that the distance between the coil centers is equal to theaverage diameter of the coils. The barriers 18a and 20a between thechambers 12a, 14a, and 16a are made of non-magnetic material, such aswatercooled copper.

The Helmholtz coil system of FIGURE 3 has two main advantages over theferromagnetic system shown in FIG- URE 1. First, the magnetic lines haveno curvatures at the apertures but continue in straight parallel lines,unaffected by the presence of the orifice or its thickness. Thus, inFIGURE 3, the electrons do not traverse a re gion where the magneticlines of flux are not parallel but are divergent on one side of theaperture and convergent on the other side. The non-parallel lines offlux may unequally aifect the electrons in the beam, depending on theposition of the electrons in the aperture. Secondly, the inductance ofthe air core coils is very much less than that of the iron core coils.Hence, the magnetic field intensity may be changed at a higher rate, asby means of a servo system, to follow any changes in high voltage in thegun cathode to keep the beam of electrons always focused on theaperture.

As can be seen from the above, an electron beam apparatus is providedwhich permits efficient use of high perveance electron guns whichnormally produce unusable divergent beams. Also, relatively inexpensivepumps may be employed in the apparatus to maintain the required vacuumin the system.

It should be realized that while one electron gun is shown in thedrawings, more than one electron gun may be provided along with theassociated apertures. Also, as previously indicate-d, in certainapplications it may be desirable to only use one barrier, that is, abarrier to separate the enclosure into only two chambers, a gun chamberand a treating chamber. In other apparatus it may be desirable toprovide more than three chambers. Various other changes andmodifications may be made in the above described electron beam apparatuswithout deviating or departing from the spirit or scope of the presentinvention.

Various features of the present invention are set forth in theaccompanying claims.

What is claimed is:

1. An electron beam apparatus for treating material comprising anair-tight enclosure, a barrier in said enclosure for dividing the sameinto two chambers, said barrier having an elongated aperture therein, anelongated electron emitting surface in one of said chambers, saidsurface being parallel to and aligned with said aperture, said aperturebeing of substantially the same size as said surface, means fordirecting said electrons at said aperture, means for establishing auniform magnetic field having parallel lines of flux from the emittingsurface to the aperture whereby electrons from said surface follow ahelical path to said aperture, the distance between the aperture and theemitting surface being substantially equal to an integral number ofrevolutions of the helical path, and means connected to said chambersfor evacuating the same to maintain a pressure differential on oppositesides of the barrier, the narrow dimension of the aperture being lessthan the mean free path of the gas molecules in the chambers.

2. An electron beam apparatus for treating material comprising anair-tight enclosure, a pair of spaced barriers in said enclosure fordividing the same into three chambers, each of said barriers having anelongated aperture therein, an elongated electron emitting surface in afirst of said chambers, said surface and each of said apertures beingparallel, of substantially the same size, and in line, means fordirecting said electrons at said aperture, means for establishing auniform magnetic field having parallel lines of fiux from the emittingsurface to the apertures whereby electrons from said emitting surfacefollow a helical path, the distance between the apertures and betweenthe aperture and said surface being substantially equal to an integralnumber of revolutions of the helical path, and means connected to eachof said chambers for evacuating the same to maintain a pressuredifferential on opposite sides of the barrier, the narrow dimension ofthe aperture being less than the mean free path of the gas molecules inthe chambers.

3. An electron beam apparatus for treating material comprising anair-tight enclosure, a barrier in said en;

closure for dividing the same into two chambers, said barrier having anelongated aperture therein, an elongated electron emitting surface inone of said chambers, said surface being parallel to and aligned withsaid aperture, means for directing said electrons at said aperture,means for establishing a uniform magnetic field having parallel lines offlux from the emitting surface to the aperture whereby electrons fromsaid surface follow a helical path to said aperture, the distancebetween the aperture and the emitting surface being substantially equalto an integral number of revolutions of the helical path, and meansconnected to said chambers for evacuating the same, the pressuredifference on the opposite sides of the barrier being of the order ofbetween 10 to 1 and to l.

4. An electron beam apparatus comprising an air-tight enclosure, a pairof spaced generally parallel barriers in said enclosure for dividing theenclosure into three chambers, means for evacuating each of saidchambers, each of said barriers being formed at least in part by a plateof ferromagnetic material, a third plateof ferromagnetic material in afirst of said compartments, said third plate being spaced from andgenerally parallel to said first mentioned plates, a Pierce typeelectron gun extending through an aperture in said third plate, said gunproducing an elongated beam of electrons which beam is directed at saidfirst mentioned plates, each of said first mentioned plates having anelongated aperture substantially the same size as the elongated beam ofelectrons, said elongated apertures being in alignment with and parallelto said elongated beam of electrons, the mean free path of gas moleculesin said aperture closer to said gun being greater than the width of saidaperture, and a plurality of cores extending between each pair ofplates, each of said cores having a winding thereon, whereby whencurrent is passed through said winding a uniform magnetic field is setup between said plates and electrons in said beam follow a helical pathto said elongated apertures, said elongated apertures being spaced fromeach other and said gun being spaced from the adjacent elongate-daperture by a distance equal to an integral number of revolutions of thehelical path.

5. An electron beam apparatus for treating material comprising anvair-tight enclosure, three equal, fiat, circular air core coilsdisposed in spaced coaxial relationship in said enclosure, the spacingbetween said coils being equal to the diameter of one of the coils, saidcoils being connected in series so as to provide a uniform field withinsaid coils, a barrier in said chamber at each of two of said coils fordividing the enclosure into three chambers, an elongated aperture ineach of said barriers, a

Pierce type electron gun disposed within said remaining coil, said gunproducing an elongated electron beam and directing the same toward saidapertures, whereby electrons in said beam follow a helical path to saidapertures, said apertures and said beam being substantially of the samesize, aligned and parallel to each other, the spacing between theapertures and between the gun and 8 the adjacent aperture being equal toan integral number of revolutions of said helix, and means forexhausting each of said compartments, the width of the aperture closerto said gun being less than the mean free path of gas molecules in thataperture.

References Cited by the Examiner UNITED STATES PATENTS 1,941,157 12/1933Smith 313-84 X 2,072,658 3/1937 Von Bronk 31386 X 2,234,281 3/1941 Ruska313-84 X 2,266,218 12/1941 Krause 313-84 X 2,293,567 8/1942 Skellett31386 X 2,369,782 2/1945 Hillier 31384 2,429,558 10/1947 Marton 31384 X2,841,726 7/1958 Knechtli 3137 3,150,256 9/1964 Wilska 31384 X JAMES W.LAWRENCE, Primary Examiner.

R. SEGAL, Assistant Examiner.

1. AN ELECTRON BEAM APPARATUS FOR TREATING MATERIAL COMPRISING ANAIR-TIGHT ENCLOSURE, A BARRIER IN SAID ENCLOSURE FOR DIVIDING THE SAMEINTO TWO CHAMBERS, SAID BARRIER HAVING AN ELONGATED APERTURE THEREIN, ANELONGATED ELECTRON EMITTING SURFACE IN ONE OF SAID CHAMBERS, SAIDSURFACE BEING PARALLEL TO AND ALIGNED WITH SAID APERTURE, SAID APERTUREBEING OF SUBSTANTIALLY THE SAME SIZE AS SAID SURFACE, MEANS FORDIRECTING SAID ELECTRONS AT SAID APERTURE, MEANS FOR ESTABLISHING AUNIFORM MAGNETIC FIELD HAVING PARALLEL LINES OF FLUX FROM THE EMITTINGSURFACE TO THE APERTURE WHEREBY ELECTRONS FROM SAID SURFACE FOLLOW AHELICAL PATH TO SAID APERTURE, THE DISTANCE BETWEEN THE APERTURE AND THEEMITTING SURFACE BEING SUBSTANTIALLY EQUAL TO AN INTEGRAL NUMBER OFREVOLUTIONS OF THE HELICAL PATH, AND MEANS CONNECTED TO SAID CHAMBERSFOR EVACUATING THE SAME TO MAINTAIN A PRESSURE DIFFERENTIAL ON OPPOSITESIDES OF THE BARRIER, THE NARROW DIMENSION OF THE APERTURE BEING LESSTHAN THE MEAN FREE PATH OF THE GAS MOLECULES IN THE CHAMBERS.